Background

A corn (also termed clavus) is a thickening of the skin due to intermittent pressure and frictional forces. These forces result in hyperkeratosis, clinically and histologically. The extensive thickening of the skin in a corn may result in chronic pain, particularly in the forefoot; in certain situations, this thickening may result in ulcer formation. The word clavus has many synonyms and innumerable vernacular terms, some of which are listed in the Table below; these terms describe the related activities that have induced clavus formation.

Synonyms for clavus include callosity, a hyperkeratotic response to trauma; corn, heloma, or a circumscribed hyperkeratotic lesion that may be hard (ie, heloma durum) or soft (ie, heloma molle); and callous, callus, or a diffusely hyperkeratotic lesion. Localized callosities of the soles, which do not resolve, are termed plantar callus, heloma, tyloma, keratoma, or plantar corn. When callosities occur over one or more lateral metatarsals, they are termed intractable plantar keratoses.
[1]

Corns are often seen in athletes and in patient populations exposed to uneven friction from footwear or gait abnormalities, including elderly persons, diabetic patients, and amputees.
[2] Abnormal foot mechanics, foot deformities, high activity level, and more serious conditions such as peripheral neuropathy also contribute to the formation of corns.
[3] Corns are associated with considerable morbidity secondary to pain; fortunately, many treatment and preventative options are available that provide a high rate of mitigation.
[4]

Clinically, all these lesions look like hyperkeratotic or thickened skin. Maceration and secondary fungal or bacterial infections are a common overlying feature in heloma molle and diabetes. Plantar helomas tend to have a central keratin plug, which, when pared, reveals a clear, firm, central core. The most common sites for clavus formation are the feet, specifically the dorsolateral aspect of the fifth toe for heloma durum, in the fourth interdigital web of the foot for heloma molle, and under the metatarsal heads for calluses. Clinically, three types of corns have been described. The first is a hard corn, or heloma durum, notable for its dry, horny appearance. It is found most commonly over the interphalangeal joints. The second is a soft corn, or heloma molle, described as such because of its macerated texture secondary to moisture. It is generally found in interdigital locations.
[5, 6] The third type is a periungual corn, and this type occurs near or on the edge of a nail.
[7] Note the image below.

Hard corn over the proximal interphalangeal joint of second toe. Courtesy of James K. DeOrio, MD.

Corns are often misdiagnosed as calluses, which are also hyperkeratotic skin lesions resulting from excess friction. However, calluses develop from forces distributed over a broad area of skin, whereas corns develop from more localized forces.
[8] Calluses are often considered desirable for some activities (eg, gymnastics, weightlifting), and they lack a central core, which is characteristically revealed in corns upon removal of the upper hyperkeratotic layer of skin. Corns can occur within an area of callus,
[9] such as on the plantar surface. Note the image below.

Calluses on the palmar surface of the hands of a body builder. Courtesy of James K. DeOrio, MD.

Pathophysiology

Corns are the result of mechanical trauma to the skin culminating in hyperplasia of the epidermis. Most commonly, friction and pressure between the bones of the foot and ill-fitting footwear cause a normal physiological response—proliferation of the stratum corneum. One of the primary roles of the stratum corneum is to provide a barrier to mechanical injury. Any insult compromising this barrier causes homeostatic changes and the release of cytokines into the epidermis, stimulating an increase in synthesis of the stratum corneum. When the insult is chronic and the mechanical defect is not repaired, hyperplasia and inflammation are common.
[26] With corns, external mechanical forces are focused on a localized area of the skin, ultimately leading to impaction of the stratum corneum and the formation of a hard keratin plug that presses painfully into the papillary dermis, which is known as a radix or nucleus.
[6, 8]

The shape of the hands and feet are important in corn (clavus) formation. Specifically, the bony prominences of the metacarpophalangeal and metatarsophalangeal joints often are shaped in such a way as to induce overlying skin friction. As corn formation ensues, friction against the footwear is likely to perpetuate hyperkeratosis. Repetitive motion can produce callosities, as would be seen in musicians.
[22]

Toe deformity, including contractures and claw, hammer, and mallet-shaped toes, may contribute to pathogenesis. Deformity of the feet from underlying conditions such as rheumatoid arthritis can contribute to clavus formation.
[23] Bunionettes, ie, callosities over the lateral fifth metatarsal head, may be associated with neuritic symptoms due to compression of the underlying lateral digital nerves. Furthermore, Morton toe, in which the second toe is longer than the first toe, occurs in 25% of the population; this may be one of the most important pathogenic factors in a callus of the common second metatarsal head, ie, an intractable plantar keratosis.

Long-term or repetitive motion may also induce clavus formation, as is seen in computer users and text messengers (ie, "mousing" callus).
[24] Callosities can also form from excessive leg crossing.
[25]

A 2005 study conducted by Menz et al reported that in older populations, plantar pressures are significantly higher under callused regions of the foot.
[28] These data support the idea that increased pressures are related to a hyperkeratotic response and that the target for treatment should be eliminating excess pressures on the foot.

Faulty mechanics play a role. Irregular distribution of pressure and repetitive motion injury (especially in athletes) are believed to be the main inciting causes; however, inappropriately shaped or constrictive footwear in the presence of bony prominences (eg, talar bone prominences
[35] ) may exacerbate corn formation. Furthermore, some disorders may alter the shape or sensation of the soles of the feet. Bony prominences and faulty foot mechanics then allow clavus formation to continue.
[36, 37, 38, 39, 40]

Rheumatoid arthritis
[41] : About 17% of patients with rheumatoid arthritis present with intractable foot pain. Chronic arthritis leads to foot deformities and consequent callus formation. Bleeding into callosities in patients with rheumatoid arthritis may be a sign of rheumatoid angiitis.

Diabetes mellitus with associated peripheral neuropathy
[42] : In patients with diabetes, chronic callosities in the presence of neurovascular deterioration may lead to ulcerations and superinfections.

Epidemiology

Frequency

United States

Corns are one of the most common foot conditions in the United States, particularly amongst older patients. It is a common disorder because of the frequency of usage of occlusive footwear and participation in repetitive activities, such as running.

International

Corns are common worldwide. Any weight-bearing human is susceptible to the development of corns.

Race

An epidemiological study evaluating the prevalence of foot conditions amongst a diverse sample of adults from the northeastern United States revealed a significant difference in rates of corns amongst ethnic groups. African Americans had a significantly higher rate of corns and calluses compared with non-Hispanic white and Puerto Rican participants (70% vs 58% vs 34.1%).
[43]

Sex

Amongst elderly populations, both men and women have been reported to wear shoes too narrow for their feet. Women have been reported to wear shoes that are also shorter than their feet. Both narrow and short footwear can lead to the development of corns, in addition to foot deformities.
[44] They are more common in women than in men because of this use of occlusive and poorly fitted footwear.

Age

Hyperkeratotic lesions of the foot (including corns and calluses) have been reported to affect 20-65% of people aged 65 or older.
[43, 45, 46]

Anyone can have a clavus, but most individuals acquire the risk factors for clavus formation after puberty because of the onset of traumatic footwear use, repetitive motion injuries, and progressive foot deformities.

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Prognosis

Recurrence is common. The most common symptoms associated with corns are pain upon ambulation and restriction of activity secondary to pain. Corns are generally not associated with mortality; however, recognizing the potential for a maltreated corn, soft corns in particular, to develop into a life-threatening secondary infection (bacterial or fungal) is important in patients with diabetes mellitus or immunosuppression.

The prognosis depends on the underlying cause of the callous formation and whether interventions can successfully be introduced to eliminate the repetitive motion. Chronic clavus generally occurs because of the difficulty in removing inciting factors in most situations. Extensive thickening of the skin may result in chronic pain, particularly in the forefoot; in certain situations, ulcer formation may result. Clavus may be a sign of underlying neuropathy due to diabetes or neuroborreliosis, or owing to the deformities of rheumatoid arthritis. In the case of neuropathy, a clavus may hide ulceration or denote abnormal neurovasculature of the feet. In the case of rheumatoid arthritis, corns may enhance the pain of deformed joints.

Patient Education

Patients must be taught to wear less traumatic footwear, such as shoes with a wide toe space. Using inner soles, lowering the heel (if second metatarsal head lesions are present), and preventing the repetitive injuries that cause occupational clavus formation may be helpful. Review of proper footwear and trauma reduction may reduce disease severity over time.

Richard K Scher, MD Adjunct Professor of Dermatology, University of North Carolina at Chapel Hill School of Medicine; Professor Emeritus of Dermatology, Columbia University College of Physicians and Surgeons

The authors and editors of Medscape Reference gratefully acknowledge the contributions of previous authors Ali Hendi, MD; Douglas W. Kress, MD; and Roger Patrick, MD, to the development and writing of this article.